Byung Chul Jeong

A repressor complex, AP4 transcription factor and geminin, negatively regulates expression of target genes in nonneuronal cells

The transcription of neuron-specific genes must be repressed in nonneuronal cells. REST/NRSF is a transcription factor that restricts the expression of many neuronal genes through interaction with the neuron-restrictive silencer element at the promoter level. PAHX-AP1 is a neuronal gene that is developmentally up-regulated in the adult mouse brain but that has no functional NRSE motif in its 5′ upstream sequence. Here, we report that the transcription factor AP4 and the corepressor geminin form a functional complex in which SMRT and histone deacetylase 3 are recruited. The functional complex represses PAHX-AP1 expression in nonneuronal cells and participates in regulating the developmental expression of PAHX-AP1 in the brain. This complex also serves as a transcriptional repressor of DYRK1A, a candidate gene for Down’s syndrome. Furthermore, compared with that in normal fetal brain, the expression of AP4 and geminin is reduced in Down’s syndrome fetal brain at 20 weeks of gestation age, at which time premature overexpression of dual-specificity tyrosine-phosphorylated and regulated kinase 1A (DYRK1A) is observed. Our findings indicate that AP4 and geminin act as a previously undescribed repressor complex distinct from REST/NRSF to negatively regulate the expression of target genes in nonneuronal cells and suggest that the AP4–geminin complex may contribute to suppressing the precocious expression of target genes in fetal brain.

The Orphan Nuclear Receptor Estrogen Receptor-related Receptor γ Negatively Regulates BMP2-induced Osteoblast Differentiation and Bone Formation

Estrogen receptor-related receptor γ (ERRγ/ERR3/NR3B3) is a member of the orphan nuclear receptor with important functions in development and homeostasis. Recently it has been reported that ERRα is involved in osteoblast differentiation and bone formation. In the present study we examined the role of ERRγ in osteoblast differentiation. Here, we showed that ERRγ is expressed in osteoblast progenitors and primary osteoblasts, and its expression is increased temporarily by BMP2. Overexpression of ERRγ reduced BMP2-induced alkaline phosphatase activity and osteocalcin production as well as calcified nodule formation, whereas inhibition of ERRγ expression significantly enhanced BMP2-induced osteogenic differentiation and mineralization, suggesting that endogenous ERRγ plays an important role in osteoblast differentiation. In addition, ERRγ significantly repressed Runx2 transactivity on osteocalcin and bone sialoprotein promoters. We also observed that ERRγ physically interacts with Runx2 in vitro and in vivo and competes with p300 to repress Runx2 transactivity. Notably, intramuscular injection of ERRγ strongly inhibited BMP2-induced ectopic bone formation in a dose-dependent manner. Taken together, these results suggest that ERRγ is a novel negative regulator of osteoblast differentiation and bone formation via its regulation of Runx2 transactivity.

Brain-specific angiogenesis inhibitor 2 regulates VEGF through GABP that acts as a transcriptional repressor.

Previously, we reported that decreased brain‐specific angiogenesis inhibitor 2 (BAI2) induced increased VEGF expression. The regulatory mechanisms for this process are not understood. Here we show that GA‐binding protein gamma (GABPγ) associates with the cytoplasmic domain of BAI2, and GABPα/γ or GABPα/β works as a transcriptional repressor of VEGF in SHSY5Y cells. Transcriptional activity of wild‐type VEGF promoter was significantly increased in anti‐sense BAI2‐transfected cells, but not that of VEGF promoter harboring mutated GABP sites. In in vivo focal cerebral ischemia model, the decrease in BAI2 accompanied by decreased GABPα and GABPγ elicited increased VEGF expression before the onset of HIF‐1α. Our results point out that BAI2 controls VEGF transcription through GABP under normal conditions and cerebral ischemia.

The Orphan Nuclear Receptor SHP Is a Positive Regulator of Osteoblastic Bone Formation

The orphan nuclear receptor small heterodimer partner (SHP; NR0B2) interacts with a diverse array of transcription factors and regulates a variety of cellular events such as cell proliferation, differentiation, and metabolism. However, the role of SHP in bone formation has not yet been elucidated. SHP expression is significantly increased during osteoblast differentiation, and its expression is partially regulated by bone morphogenetic protein 2 (BMP‐2), which plays an important role in bone formation. In our study, inhibition of SHP expression significantly repressed BMP‐2‐induced osteoblast differentiation and ectopic bone formation. In accordance with these in vitro and in vivo results, osteoblast differentiation in SHP−/− mice primary osteoblasts was significantly repressed, and the mice showed decreased bone mass resulting from decreased numbers of osteoblasts. Finally, SHP physically interacts and forms a complex with runt‐related transcription factor 2 (Runx2) on the osteocalcin gene promoter, and overexpression of SHP increased Runx2 transactivity via competition with histone deacetylase 4 (HDAC4), an enzyme that inhibits DNA binding of Runx2 to its target genes. Taken together, these results indicate that SHP acts as a novel positive regulator of bone formation by augmenting osteoblast differentiation through regulation of the transcriptional activity of Runx2.

Cyclic AMP response element-binding protein H (CREBH) mediates the inhibitory actions of tumor necrosis factor α in osteoblast differentiation by stimulating Smad1 Degradation

Background: Severe inflammatory reactions delay wound healing of bone. Results: Tumor necrosis factor α (TNFα) inhibition of osteoblast differentiation is associated with increased cAMP response element-binding protein H (CREBH) and Smurf1 expression. Conclusion: CREBH mediates the inhibitory actions of TNFα in bone regeneration. Significance: CREBH is identified as a new mediator of inflammation-dependent bone degradation and a potential therapeutic target. Endoplasmic reticulum (ER) stress transducers, such as old astrocyte specifically induced substance (OASIS) and activating transcription factor 6 (ATF6), which are induced by bone morphogenetic protein 2 (BMP2), regulate bone formation and osteoblast differentiation. Here, we examined the role of cAMP response element-binding protein H (CREBH), a member of the same family of ER membrane-bound basic leucine zipper (bZIP) transcription factors as OASIS and ATF6, in osteoblast differentiation and bone formation. Proinflammatory cytokine TNFα increased CREBH expression by up-regulating the nuclear factor-κB (NF-κB) signaling pathway in osteoblasts, increased the level of N-terminal fragment of CREBH in the nucleus, and inhibited BMP2 induction of osteoblast specific gene expression. Overexpression of CREBH suppressed BMP2-induced up-regulation of the osteogenic markers runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), and osteocalcin (OC) in MC3T3-E1 cells and primary osteoblasts, as well as BMP2-induced ALP activity and OC protein production. In contrast, knockdown of CREBH attenuated the inhibitory effect of TNFα on BMP2-induced osteoblast differentiation. Mechanistic studies revealed that CREBH increased the expression of Smad ubiquitination regulatory factor 1 (Smurf1), leading to ubiquitin-dependent degradation of Smad1, whereas knockdown of CREBH inhibited TNFα-mediated degradation of Smad1 by Smurf1. Consistent with these in vitro findings, administration of Ad-CREBH inhibited BMP2-induced ectopic and orthotopic bone formation in vivo. Taken together, these results suggest that CREBH is a novel negative regulator of osteoblast differentiation and bone formation.